This invention relates to a multiple access communication system between a master station and slave stations and, more particularly, to the one for processing various kinds of data of different traffic characteristics.
Recently interactive multi-media networks such as CATV have been adapting bi-directional communication allowing for data transmission between a master station and slave stations interactively. Generally the data transmission from the master station to the slave station has been conducted on a broadcasting channel by which the same data are transmitted to all the slave stations concurrently. While the data transmission from the slave station to the master station has been conducted on a multiple access channel by which data are time division multiplexed for transmission therebetween. In a certain system, the master station designates a slave station which is expected to transmit the data by a polling signal. Responding to the receipt of the polling signal, the designated slave station transmits the data to the multiple access channel.
In case 3 slave stations are expected to transmit data sequentially, the master station designates the slave station one after another and transmits a polling signal. The length of data which can be sent by each slave station is designated by the polling signal output from the master station. The slave station transmits bandwidth request information containing the designated data length and data length required to be sent responding to the receipt of the next polling signal. Based on the bandwidth request information from the slave station, the master station determines the data length which will be designated in the next polling signal.
The time interval for transmitting the polling signal to the slave station is equal to the data length which is expected to be transmitted from each slave station. This intends to multiplex the respective data on the same channel most efficiently without leaving any gap therebetween. Each slave station starts transmitting the data immediately upon receiving the polling signal. The master station transmits the next polling signal without waiting for the data from slave unit.
Recently packet communication has also been increasingly adapted. In this communication, transmission data are divided into units of packets transmittable in a predetermined size. Asynchronous Transfer Mode (hereinafter referred to as ATM) system is especially suitable for transmitting and receiving sound and video information requiring strict punctuality. In the ATM communication system, all the information uses cells to transmit various data of traffic characteristics. For example, Constant Bit Rate (CBR) data service requires transmission of a predetermined length of data at a given time interval. Another service is a Variable Bit Rate (VBR) data service which allows for variation in transmission data length but requires keeping the average length of data transferred to a predetermined value or more. Also Available Bit Rate (ABR) data service allows for transmission of any data length.
In the above-described conventional multiple access communication system, the interval for polling signal transmission by the master station is defined by the length of data expected to be transmitted from the slave station. The length of data sent from the slave station varies with the bandwidth request information from the slave station. Supposing that cyclic polling is conducted to 3 slave stations, each cycle for a round of polling varies with the amount of data sent from the slave station. So in this system, the CBR data requiring constant cycle and data length cannot be transmitted. Furthermore, the conventional multiple access communication system is not designed to retain average length of data sent from the respective slave stations. This indicates that the VBR data requiring a constant average length of the data cannot be used. The prior art system has difficulty in transmitting the ATM mode communication data with adequate traffic characteristics where CBR, VBR and ABR are mixed therein.